Heinz Burtscher

POLYCYCLIC AROMATIC HYDROCARBONS AND COMBUSTION AEROSOL PHOTOEMISSION

Abstract An experimental investigation was conducted to explain the physical chemistry of aerosol photoemission (APE) from combustion sources. Strong correlations between APE and total polycyclic aromatic hydrocarbons (TPAH) in particulate matter were observed for aerosols from both oil stove and automobile exhaust. Sampling and analytical procedures were developed to allow detection of PAHs ranging in molecular weight (MW) from phenanthrene (MW 178) to coronene (MW 300) in oil stove exhaust after short sampling periods. Small variations of the oil stove PAH profile with Bacharach number were observed. Interactions between adsorbed polycyclic aromatic hydrocarbons PAHs and particle surface are proposed as the most likely cause of environmental APE after irradiation at 4.9 eV.

Photoelectric quantum yield of nanometer metal particles

The absolute photoelectric quantum yield of nanometer metal particles (Ni, Pd, Cu, Ag, Au) is measured from threshold up to photon energies hν of 10 eV. The particles are produced and measured in ultrapure helium at atmospheric pressure. Yield Y and photothreshold φ are compared to bulk data. The shape of Y(hν) is similar for particle and bulk with all investigated materials except Ag. All metals have in common that the particle yield is larger by a factor ≊100 compared to the bulk yield. Apart from electrostatic corrections due to the image and Coulomb potential, the photoelectric threshold is identical for the bulk and the particle.

The effect of van der Waals forces on aerosol coagulation

Abstract The enhancement α of the coagulation coefficient by dispersion forces is derived for spherical aerosol particles using the comprehensive theories by D. Langbein. For small radii R1, R2 (R1, R2 ≲ 10−5 cm) only nonretarded dispersion forces are significant and α depends on R 1 R 2 and the Lifshitz-van der Waals constant h ω − only. For large particle radii (R1, R2 ≳ 10−3 cm), α is unity. In the transition region both nonretarded and retarded forces have to be considered. Values for α(R1, R2) applicable to nonmetallic particles in air are listed for 2 × 10−6 cm α( h ω − ) is discussed. The results differ considerably from an earlier calculation by Twomey [“Atmospheric Aerosols.” Elsevier, Amsterdam/New York, 1977] , in which α is overestimated.

Photoelectric charging of ultrafine volcanic aerosols: Detection of Cu(I) as a tracer of chlorides in magmatic gases

Volcanic gases contain ultrafine aerosol particles in the nanometer size range; typical concentrations are 105 to 106 cm−3. Photoelectric charging of particles (PCP) is an in situ method for the material specific detection of very small particles in a gas. Field studies at degassing lava flows of Mount Etna, Sicily, and Kilauea, Hawaii, show that the chemistry of the ultrafine aerosols depends strongly on the degassing state of the lava. Heating of a relatively undegassed lava sample in the laboratory reveals the chemical nature of the particles that form by nucleation and condensation in the cooling gas. In the initial stages of degassing, the particles are mainly NaCl and KCl nanocrystals that contain iron oxide and copper chloride. Cu is in the monovalent state, which is stable even in an oxidizing environment due to a redox mechanism with the Fe ions. The fraction of Cu(I) in the aerosols is determined by PCP. The evolution of the Cu(I) fraction is considered an effective tracer of chlorides in the magmatic gases and thereby of magma degassing. The PCP technique allows a fingerprint to be obtained of the magmatic gas by an aerosol measurement in the diluted plume.

Measurement of size distribution and photoelectric activity of particles in a gas diffusion flame

The aerodynamic diameter D of the particles in a gas diffusion flame and the concentration of particle-bound polyaromatic hydrocarbons (PAH) are determined by in situ aerosol measurement techniques. Therefore a small gas volume is extracted at different heights from the combustion zone through a thin quartz capillary. By rapid cooling on expansion and ∼ 600-fold dilution with air at ambient temperature the physical and chemical processes affecting the particles are quenched. We find 5⩽D⩽10 nm, and D increases with increasing height above the burner. The number concentration of the particles has, however, a maximum in the middle of the flame, indicating particle formation and growth by condensation and agglomeration up to the middle and annihilation by burning in the upper part of the flame. Photoelectric charging of the particles indicates that PAHs are present in or on the particles everywhere, yet PAH concentration is higher at lower heights and also with smaller particles. Although the total mass of particles in the diffusion mode of the flame is much higher compared to the premixed mode, the size distribution is very similar in both cases. This indicates that the size of the particles and total particulate mass are not directly related.

Analysis of combustion products using time-of-flight mass spectrometry

Time of flight mass spectrometry (TOF-MS) combined with resonant two-photon ionization is applied to detect aromatic hydrocarbons in combustion products. The sample is introduced through a heated capillary into the supersonic jet of a TOF-MS. For ionization, a tunable, frequency-doubled dye-laser is used. The inlet system allows a direct measurement of samples at atmospheric pressure. The resonant ionization can distinguish structural isomers without preceding chromatography. The short response time of the system allows for example on-line monitoring of combustion emissions. This is demonstrated by thermodesorption experiments of anthracene from activated charcoal and from diesel soot. A significant higher desorption temperature from charcoal than from diesel soot is found. In addition, examples showing selective measurements of benzene, toluene and three dimethyl-benzene isomers in the exhaust gas of a gasoline engine are presented.

Photoelectric charging and detection of ultrafine particles

Abstract Ultrafine aerosol particles can be photoelectrically charged by irradiation with u.v.-light of photon energy below the threshold for O3 and NO x production. A concentration of some hundred particles cm−3 of diameter 〈 20 nm was determined in suburban air by measuring the product of concentration n( cm −3 ) and mean charge per particle q . The relative concentration of the chargeable particles is high during rain periods and low during sunny periods. A consistent maximum is observed when the base of the morning inversion reaches the hill-top measuring site. The particles appear to be organic in nature with photoelectrically active species on their surfaces.

Metallic cluster photoemission

The photoelectric yield of metallic clusters (Ag, Au) in the energy range between threshold and 10eV has shown a completely different trend between the two metals and strong deviations from the bulk for Ag particles. On the contrary, good agreement has been obtained for both metals with the Fowler-Nordheim law near threshold. Attempts to explain the previous discrepancies with calculations of the density of states did not clarify the experimental differences observed. We have approached this problem from another point of view applying the theory of diffraction, the absorption of the radiation beam and the electron mean free path inside the cluster. Good agreement between the experimental results and the calculated curves has been obtained over the entire range of the s-band both for the Ag and Au particles.